Remove dead code: domain_event.py + 39 overridden templates

- Delete shared/models/domain_event.py (table dropped, model orphaned)
- Delete 39 shared templates that are overridden by app-local copies:
  - 8 blog overrides (blog/_action_buttons, post/_meta, etc.)
  - 27 events overrides (calendar/*, day/*, entry/*, post_entries/*)
  - 4 market overrides (market/index, browse/_oob_elements, etc.)

These shared copies were never served — Quart loads app-level
templates first, so the app-local versions always win.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -12,7 +12,7 @@ shared/
   models/                   # Canonical domain models
     user.py                 # User
     magic_link.py           # MagicLink (auth tokens)
-    domain_event.py         # DomainEvent (legacy — being removed)
+    (domain_event.py removed — table dropped, see migration n4l2i8j0k1)
     kv.py                   # KeyValue (key-value store)
     menu_item.py            # MenuItem (deprecated — use MenuNode)
     menu_node.py            # MenuNode (navigation tree)